28 LIFE ON THE EARTH. 



nervous cord, ganglionated at intervals, proceeding 

 along the body. Symmetry bilateral. 



III. Mollusca. Body not jointed or ringed across : ner- 



vous cord encircling the alimentary canal, and rami- 

 fying through the body. Symmetry bilateral or 

 spiral. 



IV. Radiata. Nervous system absent or reduced to a 



ring round the alimentary canal with few radiating 

 threads. Symmetry radiate round an axis. 



There are some forms of plants and animals so 

 slightly adapted to special purposes of life, as far as 

 we have yet discovered, that life may be thought to 

 reside in them only in a general form; the type of 

 which if that can be called typical which seems 

 rather to be marked by absence of all but elementary 

 organization may be called cellular or rudimentary. 

 This group, however, is but provisional, and will pro- 

 bably be hereafter better divided according as the 

 nutritive and reproductive systems are more surely 

 analysed by the microscope. 



Under the great types of vegetable and animal 

 structure which have been mentioned, naturalists find 

 it convenient to adopt many classes, orders, families, 

 genera, species and varieties. These are not so set- 

 tled in any case as to be altogether free from change 

 by fresh inquiries and discoveries: they have been 

 augmented and modified by the discoveries of palae- 

 ontology; which have filled several void spaces in 



